1. Field of the Invention
The present invention relates generally to the field of predicting sonar performance, and more particularly to systems and methods of predicting the performance of high-frequency, passive sonar arrays in the open ocean and in shallow water coastal sites.
2. Description of the Related Art
Sonar, a well known technology in which sound waves are used for detecting objects, has found extensive application in maritime environments. Sonar systems can be broadly divided into two types: active sonar and passive sonar. An active sonar system includes a sound source, which emits bursts of sound, and an array of receivers which listen for the reflection of the emitted sound from an object. A passive sonar system has no sound source and relies on its array of receivers to detect sound emitted by the object being sought. A passive sonar system can only detect objects emitting sounds louder than the ambient noise of the environment, but has the major advantage over active sonar systems of not revealing its presence or location.
The choice of what frequency of sound waves to listen for in a passive, underwater sonar system is to a large extent a tradeoff between range and cost. The greatest range is obtained by listening for the lowest frequency sound waves, as the distance sound propagates in water is inversely proportional to the frequency of the sound wave, primarily because of lower absorption. However, the size and cost of sonar arrays is also generally inversely proportional to the frequency of the sound waves being detected.
Throughout the cold war, a major objective of US Naval sonar was to conduct long range, deep ocean surveillance. A majority of the passive sonar systems designed and used by the US Navy during that period were used for locating objects at a range of several hundred miles in deep ocean conditions, and these systems operated by detecting low frequency sound waves, in the range of 100–300 Hz. At these frequencies, the dominant source of ambient noise is due to distant shipping rather the noise generated by the ocean surface. Given the contribution of many distant ships to the ambient field in deep ocean areas, a common assumption is that the ocean's ambient noise at low frequencies is isotropic, i.e. that it is the same in all directions.
The end of the cold war has lead to an increased need for detecting objects in shallow water at much shorter ranges, typically of the order of 1 to 50 miles. At these ranges, higher frequency sonar systems, operating in the 1–20 kHz range, have significant potential advantages, including reduced size and cost. However, predicting the performance of such sonar systems requires major changes in the modeling methods. Firstly, higher frequency predictions generally require ray-based rather than wave-based modeling techniques. Secondly, the databases that describe surface and bottom interactions at low frequencies may not be applicable for use at higher frequencies. Thirdly, the ambient noise at higher frequencies and in coastal areas can also be significantly anisotropic, i.e. it can vary depending on the direction in which the sonar is “looking”.
The reason for this anisotropy can be understood by considering how ocean ambient noise is produced and propagated. At these higher frequencies of 1–20 kHz, the primary sources of ambient noise is wind-wave activity. In certain locations, nearby shipping can also be a source of noise. The ambient noise level is related to how well all of these surface sound sources propagate through the water to the receiver. This propagation can be predicted if the bathymetry and the sound speed structure of the water columns in the region of interest are known. In particular, the nature and degree of interaction of the propagating sound with the ocean surface and the ocean floor dictates how the ambient noise level varies with listening angle. When listening up towards the surface, a sonar array listens directly to the primary sources of the ambient noise, resulting in high noise levels. When listening straight down, the noise level is lower because of the loss associated with the reflection off the ocean floor (also known as “bottom loss”). At angles in between, the noise level varies, depending on factors such as the receiver depth, the local bathymetry or ocean depth, sea bed reflectivity and sound speed profile. In particular, in downward refracting environments there is a minimum, or “noise notch”, of significantly quieter background noise at or near the horizontal plane. Since the ambient noise sets the limit on what other noise sources can be detected by a passive sonar system, it is necessary to accurately model that ambient noise, including its directionality, in order to provide a realistic simulation of a passive sonar system's performance. Methods exist for simulating reverberation in multi-path sonar such as, but not limited to, the system described in U.S. Pat. No. 6,002,914 issued to Weinberg on Dec. 14th, 1999 entitled “Method and Apparatus for Simulating Reverberation in a Multi-path Sonar System”, the contents of which are hereby incorporated by reference. However, there is no practical system for modeling the anisotropic ambient ocean noise in the 1–20 kHz frequency range and therefore no practical system for accurately simulating the performance of passive sonar arrays at these frequencies.
What is needed is a system and method that can automatically obtain and integrate the bathymetry and oceanographic data of a defined area with known or predicted weather, sea-surface and shipping patterns for that area, and use the integrated data to the performance of a chosen sonar system including the effects of the anisotropic wind-wave and shipping noise. Such a system would solve the problem of how to efficiently and accurately predict high-frequency sonar performance at arbitrary locations, including shallow water locations near coastal shelves.